1. Field of the Invention
The present invention relates to structure of an optical device, such as a semiconductor laser, and structure for mounting the optical device.
2. Related Art
In an optical fiber communication system, optical modules occupy a large part of the parts cost. To lower the price of an optical module, in particular, it is very significant to simplify the mounting of the optical device in the optical module. Accordingly, there have been several simple mounting methods known widely. Their mainstream is to mount a semiconductor laser junction down on a simple mount in an optical module. For example, such simple mounting structures were reported in the 1997 General Conference of the Institute of Electronics, Information and Communication Engineers, Proceedings, Lecture Nos. C3-65, CS-68, etc.
When a semiconductor laser is mounted junction down, the semiconductor laser is bonded to a simple optical device mounting plate made of Si or the like via a solder metal. In this junction down mounting, reliability of the semiconductor laser may deteriorate if reaction advances between the solder metal and the electrode metal of the semiconductor laser. This is attributable to some factors. One is stress acting in the active layer and absorption layer of the optical device as described in Japanese Patent Laid-open No. 11-87849. Another is metal diffusion into the semiconductor as described in public documents such as Journal of Electron Material, vol. 27 (1998), p. 89-95. Metal may diffuse into the semiconductor and further penetrates into the active layer, causing defects there which may deteriorate the device's characteristics.
In order to secure the life of the optical device by suppressing such reaction/diffusion between the electrode metal and the solder metal, there have been several methods proposed and known. They includes (1) suppressing the diffusion of the solder metal by inserting Ti or Pt in the electrode of the optical device and (2) suppressing reaction between the electrode of the optical device and the solder metal.
Of them, in the case of approach (1) where Ti or Pt is inserted to prevent metal diffusion, its effect may be not sufficient depending on the device structure. For example, a ridge waveguide type laser may have an uneven surface. If the surface has large up and down features, it is not possible to coat the surface with a uniform Ti or Pt layer as the case may be. If the Ti or Pi layer is locally thin or absent in a certain area, it is not possible to prevent the solder metal from penetrating through this area. Although the unevenness may be compensated for by thickening the Ti or Pt layer, reliability of the semiconductor laser may deteriorate since thickening the Ti or Pt layer causes larger stress.
In the case of approach (2), several solutions could be conceivable. For example, a widely known structure proposed in Japanese Patent Laid-open No. 11-87849 has a void in the solder metal layer formed on the mounting plate. Since the void is positioned near to the optical device's active region or absorption layer, reaction between the electrode metal of the optical device and the solder metal is suppressed. Specifically, a dielectric film is formed both on the surface of the optical device to be mounted and the solder metal surface of the mounting plate so that the electrode of the optical device does not come into contact with the solder metal. As a matter of course, this structure can effectively block metal penetration into the active region. However, the void formed near the active region produces a new problem with thermal conductivity due to lack of contact with the mounting plate.
This problem is described below.
Injecting current in a semiconductor laser generates heat. For the semiconductor laser to operate steadily, it is necessary to quickly dissipate the heat from the active region. However, forming a void above the active layer stripe, like in the example mentioned above, lowers the thermal conductivity from the active region and therefore raises the temperature of the active region of the semiconductor laser. As a result, this causes such problems as the necessity of increasing the operating current of the semiconductor laser. For practical use, the structure must not only suppress the reaction between the electrode metal of the optical device and the solder metal of the mounting plate but also have good thermal conductivity.